1. Field of the Invention
The invention relates to a sintered sprocket to be used for a sprocket of a cam shaft timing chain of an automobile and to a production method therefor.
2. Related Art
As such a sprocket, those formed into sprocket members by forging, finished by machining, and being case hardened and tempered can be exemplified. Also, to produce sintered sprockets by powder metallurgy technique for easy forming, techniques of producing sprockets made of ferrous materials containing Ni, Mo, Cu and C by warm-compacting a powder mixture to a relatively high density, sintering the green compacts, case hardening or induction hardening the sintered bodies, and then tempering the obtained bodies have been proposed (reference to Japanese Patent Application Laid-Open No. 2001-295915, page 2; hereinafter referred to as Patent Document No. 1). Furthermore, techniques involving densifying the gear teeth of sintered bodies made of ferrous materials with similar compositions by forming by rolling and then case hardening and tempering have been proposed (reference to Japanese Patent Application Laid-Open No. 2002-129295, page 2; hereinafter referred to as Patent Document No. 2).
Since a sprocket for a silent chain has a structure of interlocking gears and transmitting the rotation by the interlocking of the gears, as compared with that of a common sprocket for a roller chain, the interlocking contact pressure of the sprocket for a silent chain is high. Therefore, the sprocket for a silent chain is required to have a high density in the gears as compared with that of a sprocket for a roller chain and also to have a high hardness by hardening. The sintered sprockets described in the above-mentioned Patent Documents Nos. 1 and 2 are obtained by means of adding Ni, Mo, and Cu for increasing the material strength and hardenability, increasing the density by warm-compacting and forming by rolling, and accordingly improving the wear resistance and the fatigue strength. Accordingly, in terms of the production cost and the process simplification, it has been required to develop a technique of producing a sprocket by using a material containing fewer addition elements and employing a simple densifying means.